Fuel cells have been used as a power source in many applications including a power source for an electrical vehicle to replace internal combustion engines. In proton exchange membrane (PEM) fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. A PEM fuel cell includes a membrane electrode assembly (MEA) having a thin, proton transmissive, non-electrically conductive solid polymer electrolyte membrane with the anode catalyst on one face and the cathode catalyst on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements or bipolar plate assembly.
The fuel cell stack includes a plurality of cells stacked together in series while being separated one from the next by the bipolar plate assembly. Typically, the bipolar plate assembly includes a gas permeable, electrical conductive diffusion media and a gas impermeable, electrically conductive separator plate. The bipolar plate assembly serves several functions including (1) to distribute reactant gases across substantially the entire surface of the membrane; (2) to conduct electrical current between the anode of one cell and the cathode of the next adjacent cell in the stack; (3) to keep the reactant gases separated in order to prevent auto ignition; (4) to provide a support for the proton exchange membrane; (5) to accommodate the internal pressure loads associated with the reforming process and the external compression loads on the stack; and (6) to provide internal cooling passages to remove heat from the stack.
The separator plate includes an array of lands and grooves in the faces thereof which define a flow field for distributing the reactant gases (i.e., hydrogen and oxygen in the form of air) over the surfaces of the respective cathode and anode. The arrangement of the lands and the channels on both sides is such that the separator plate can withstand the compression loads. A piece of graphite paper or other diffusion media is placed over the flow field to prevent the MEA from collapsing down into the channel and blocking the flow of gas and to provide an electrical conduction path to the separator plate across the area of the membrane which overlays the channel.
Separator plates have been made from metal. However, the metal is relatively heavy and the corrosive environment requires that the metal plates be related with expensive coatings. Separator plates have also been fabricated from graphite which is lightweight compared to traditional metal plates, corrosion resistant and electrically conductive in the PEM fuel cell environment. However, graphite is quite brittle which makes it difficult to handle and has a relatively low electrical and thermal conductivity compared to metals. Graphite is also quite porous making it difficult and expensive to provide a thin gas impervious plate having the desired gravimetric and volumetric characteristics for a fuel cell stack.
The efficient operation of a fuel cell system depends on the ability of the fuel cell to generate a significant amount of electrical energy for a given size, weight, and cost of the fuel cell. Maximizing the electrical energy output of the fuel cell for a given size, weight, and cost is especially important in motor vehicle applications where size, weight, and cost of all vehicular components are especially critical to the efficient manufacture and operation of the vehicle. Therefore it is desirable, particularly for motor vehicle applications, to provide a fuel cell construction which will generate an increased amount of electrical energy for a given size, weight, and cost of the fuel cell.